Referring to FIG. 1, a color CRT 10 generally comprises an evacuated glass envelope consisting of a panel 12, a funnel 13 sealed to the panel 12 and a tubular neck 14 connected by the funnel 13, and electron gun 11 centrally mounted within the neck 14, and a shadow mask 16 removably mounted to a sidewall of the panel 12. A three-color phosphor screen is formed on the inner surface of a display window or faceplate 18 of the panel 12.
The electron gun 11 generates three electron beams 19a, or 19b, said beams being directed along convergent paths through the shadow mask 16 to the screen 20 by means of several lenses of the gun and a high positive voltage applied through an anode button 15 and being deflected by a deflection yoke 17 so as to scan over the screen 20 through apertures or slits 16a formed in the shadow mask 16.
In the color CRT 10, the phosphor screen 20, which is formed on the rear surface of the faceplate 18, comprises an array of three phosphor elements R, G and B of three different emission colors arranged in a cyclic order of a predetermined structure of multiple-stripe or multiple-dot shape and a matrix of light absorptive material surrounding the phosphor elements R, G and B, as shown in FIG. 2.
A thin film of aluminum 22 or an electro-conductive layer, overlying the screen 20 in order to provide a means for applying the uniform potential applied through the anode button 15 to the screen 20, increases the brightness of the phosphor screen and prevents ions in the phosphor screen from being lost and the potential of the phosphor screen from decreasing. And also, a film of resin 22'such as lacquer (not shown) may be applied between the aluminum thin film 22 and the phosphor screen 20, so as to enhance the flatness and reflectivity of the aluminum thin film 22.
In a photolithographic wet process, which is well known as a prior art process for forming the phosphor screen, a slurry of a photosensitive binder and phosphor particles is coated on the inner surface of the faceplate. It does not meet the higher resolution demands and requires a lot of complicated processing steps and a lot of manufacturing equipments, thereby requiring high cost in manufacturing the phosphor screen. In addition, it discharges a large quantity of effluent such as waste water, phosphor elements, 6th chrome sensitizer, etc., with the use of a large quantity of clean water.
To solve or alleviate the above problems, the improved process of electrophotographically manufacturing the screen utilizing dry-powdered phosphor particles is developed.
U.S. Pat. No. 4,921,767, issued to Datta at al. on May 1, 1990, discloses one method of electrophotographically manufacturing the phosphor screen assembly using dry-powdered phosphor particles through a series of steps represented in FIGS. 3A to 3E, as is briefly explained in the following.
After the panel 12 is washed, an electro-conductive layer 32 is coated on the faceplate 18 of the panel 12 and the photoconductive layer 34 is coated thereon, as shown in FIG. 3A. Conventionally, the electro-conductive layer 32 is made from an inorganic conductive material such as tin oxide or indium oxide, or their mixture, and preferably, from a volatilizable organic conductive material such as a polyelectrolyte commercially known as polybrene (1,5,-dimethyl-1,5-diaza-undecamethylene polymethobromide, hexadimethrine bromide), available from Aldrich Chemical Wisc., or another quaternary ammonium salt.
The polybrene is applied to the inner surface of the faceplate 18 in an aqueous solution containing about 10 percent by weight of propanol and bout 10 percent by weight of a water-soluble adhesion-promoting polymer (poly vinyl alcohol, polyacrylic acid, polyamides and the like), and the coated solution is dried to form the conductive layer 32 having a thickness from about 1 to 2 microns and a surface resistivity of less than about 10.sup.8 l (ohms per square unit).
The photoconductive layer 34 is formed by coating the conductive layer 32 with a photoconductive solution comprising a volatilizable organic polymeric material, a suitable photoconductive dye and a solvent. The polymeric material is an organic polymer such as polyvinyl carboazole, or an organic monomer such as n-ehtyl carbazole, n-vinyl carbazole or tetraphenylbutatriene dissolved in a polymeric binder such as polymethylpolypropylene carbonate. The photoconductive composition contains from about 0.1 to 0.4 percent by weight such dyes as crystal violet, chloridine blue, rhodamine EG and the like, which are sensitive to the visible rays, preferably rays having wavelength of from about 400 to 700 nm. The solvent for the photoconductive composition is an organic such as chlorobenzene or cyclopentanone and the like which will produce as little cross contamination as possible between the layers 32 and 34. The photoconductive layer 32 is formed to have a thickness from about 2 to 6 microns.
FIG. 3B schematically illustrates a charging step, wherein the photoconductive layer 34 overlying the electro-conductive layer 32 is positively charged in a dark environment by a conventional positive corona discharger 36. As shown, the charger or charging electrode of the discharger 36 is positively applied with direct current while the negative electrode of the discharger 36 is connected to the electro-conductive layer 32 and grounded. The charging electrode of the discharger 36 travels across the layer 34 and charges it with a positive voltage in the range from -200 to +700 volt.
FIG. 3C schematically shows an exposure step, wherein the charged photoconductive layer 34 is exposed through a shadow mask 16 by a xenon flash lamp 35 having a lens system 35' in the dark environment. In this step, the shadow mask 16 is installed on the panel 12 and the electro-conductive layer 32 is grounded. When the xenon flash lamp 35 is switched on to shed light on the charged photoconductive layer 34 through the lens system' and the shadow mask 16, portions of the photoconductive layer 34 corresponding to apertures or slits 16a of the shadow mask 16 are exposed to the light. Then, the positive charges of the exposed areas are discharged through the grounded conductive layer 32 and the charges of the unexposed areas remain in the photoconductive layer 34, thus establishing a latent charge image in a predetermined array structure, as shown in FIG. 3C. In order to exactly form light-absorptive matrices, it is preferred that the xenon flash lamp 35 travels along three positions while coinciding with three different incident angles of the three electron beams.
FIG. 3D schematically shows a developing step which utilized a developing container 35" containing dry-powdered light-absorptive or phosphor particles and carrier beads for producing static electricity by coming into contact with the dry-powdered particles. Preferably, the carrier beads are so mixed as to charge the light-absorptive particles with negative electric charges and the phosphor powders with positive electric charges, when they come into contact with the dry-powdered particles.
In this step, the panel 12, from which the shadow mask 16 is removed, is put on the developing container 35' containing the dry-powdered particles, so that the photoconductive layer 34 can come into contact with the dry-powdered particles. In this case, the negatively charged light-absorptive particles are attached to the positively charged unexposed areas of the photoconductive layer 34 by electric attraction, while the positively charged phosphor particles are repulsed by the positively charged unexposed areas but attached by reversal developing to the exposed areas of the photoconductive layer 34 from which the positive electric charges are discharged.
FIG. 3E schematically represents a fixing step by means of infrared radiation. In this step, the light-absorptive and phosphor particles attached in the above developing step are fixed together and onto the photoconductive layer 14. Therefore, the dry-powdered particles include proper polymer components which may be melted by heat and have proper adhesion.
Where the surface of the panel is flat, a conventional linear corona charger, such as those shown and described in U.S. Pat. Nos. 3,475,169, 3,515,548, and 4,386,837 issued respectively on Oct. 28, 1969, Jun. 2, 1970, and Jun. 7, 1983, can be used in the above-described charging step shown in FIG. 3B. However, where the interior surface contour of the faceplate panel is non-planar or has a certain curvature as the usual panel, the conventional linear charger will not uniformly charge the photoconductive layer and may generate deleterious arcs because the spacing between the charger and the photoconductive layer cannot be maintained uniformly.
To overcome the above problems, U.S. Pat. No. 5,132,188 discloses another corona discharge apparatus 36 having a corona charger 50 as shown in FIGS. 4 and 5.
Referring to FIG. 4, the corona discharge apparatus 36 includes a housing 38 having a faceplate panel support surface 40. A faceplate panel 12 having a conductive layer 32 and a photoconductive layer 34 coated thereon, is placed upon the support surface 40 and positioned by a plurality of panel alignment members 42, which engage the outer surface of the panel sidewall. An electrical ground contact 44, attached at one end of the housing 38, is spring biased to contact the conductive layer 32. A corona generator 46 is disposed within the housing 38. The generator 46 includes a high voltage power supply 48, which provides a corona voltage to a corona charger 50. The corona charger 50 is pivotally attached, at the center of curvature of the faceplate 12, by means of a support arm 52 to a support bar 54. The support arm 52 is connected to a motor 56 by a reciprocating drive screw 58, which causes the corona charger 50 to make multiple passes across the faceplate panel 12. The ultimate charge on the photoconductive layer 34 is determined by the number of passes across the panel which, in turn, is controlled by a timer 60 which communicates with a motor controller 62 and the high voltage power supply 48. The charging sequence is initiated from a control panel 64. An electrostatic voltage probe 84, coupled to a voltmeter 86 on the control panel 64, measures the voltage on the layer 34 at the end of the charging cycle. A probe driver 83 moves the probe 84 into proximity with the charged photoconductive layer 34.
While only one corona charger 50 is shown in FIG. 4, multiple chargers may be used.
The corona charger 50 is shown in FIG. 5. The corona charger comprises an arcuately-shaped ground electrode 66 having two parallel sides 68 and an interconnecting base 70, which form a U-shaped conductor. The sides 68 terminate in edges 72 that are rounded to suppress arcs during operation. A foil charging electrode 74 is supported, by means of an insulator 76, between the sides 68 and the base 70 of the ground electrode. The charging electrode 74 also is arcuately-shaped and, preferably, has a substantially arcuately-contoured edge 78 with a plurality of pin-type projections 80 extending therefrom. The arcuately-contoured edge 78 and sides 68 are coincident with the curvature of one axis, for example the minor axis, of the interior surface of the faceplate panel 12. The length of the support arm 52 is adjusted so that the center of curvature of the arc of the charger 50 coincides with the center of curvature of one of the axes of the panel interior surface.
In the means time, U.S. Pat. No. 5,519,217 issued to Wilbur, Jr. et al., on May 21, 1996, discloses a charging apparatus having a plurality of electrodes or blades installed on a base over the entire interior surface of the faceplate 18, detailed depiction of which is omitted in the attached drawings. In the apparatus, the focusing blades correspond to the above ground electrode, and the charging blades are disposed respectively between the adjacent focusing blades and have a plurality of serration formed at the ends thereof. The charging head moves laterally within the faceplate panel by a distance substantially equal to the periodic spacing between the charging blades, thereby providing a substantially uniform electrostatic charge to the photoconductive layer on the faceplate. Therefore, the apparatus greatly increases the charging speed or shortens the charging time without jeopardizing the uniformity of the charge applied to the photoconductive layer, thereby greatly enhancing capability in mass production.
In order to achieve uniform exposing and developing in the steps shown in FIGS. 3C and 3D, it is preferred that the photoconductive layer 34 may be uniformly charged. Further, the charging electrodes and the photoconductive layer 34 must be prevented from being damaged by arc or spark therebetween. Therefore, the above-mentioned apparatuses employ arcuately-shaped thin plates as electrodes for charging, each of the plates having a plurality of pin-type projections 80 or serration, so as to provide a stable and uniform electrostatic charge to the photoconductive layer by means of desired corona charging.
Still, it is not easy for the pin-type projections 80 or serration to cause a uniform corona discharge due to their inherent shapes. That is, the greatest discharge is generated at the distal end of each projection or each tooth of the serration, while the intensity of discharge decreases as it goes far from the distal end. This problematic discharge causes multiform exposing and developing the above exposing and developing steps, thereby forming phosphor elements multiformly even in a desired array.
Meanwhile, it is well known in the art that a wire-type corona charger generates stable and highly uniform corona discharge and exhibits superior charging efficiency relative to other types of electrodes. However, because the interior surface of the panel 12 is spheric, and moreover because the larger cathode ray tube has the more complex aspheric panel surface in which the curvature of the horizontal section is larger than that of the vertical section, it is not easy for the wire electrodes to coincide with such complex curvatures.
The present invention has been made to overcome the above described problems, and therefore ti is an object of the present invention to provide a wire-type corona charger for electrophotographically manufacturing a screen of a CRT, which can uniformly charge the photoconductive layer by generating corona discharge through wire electrodes.
It is another object of the present invention to provide a method for electrophotographically manufacturing a screen of a CRT using the wire-type corona charger.